Intravenous fluid warming system

ABSTRACT

A fluid warming device includes axially or longitudinally slidable covers that retain a removable heat exchange body in a housing in heat exchange communication with a heater assembly while allowing flow through the heat exchange body to be observed. In another aspect, a fluid warming system increases or decreases power to a heater assembly to adjust the fluid temperature to ensure that the fluid is at an appropriate temperature when it reaches the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 11/385,085, filed Mar. 21, 2006, which claims benefit of U.S.provisional patent application Ser. No. 60/663,857, filed Mar. 21, 2005.Each of the aforementioned related patent applications is hereinincorporated by reference.

BACKGROUND

Intravenous (IV) fluid warming devices are known that heat an IV fluidprior to introducing the fluid into a patient. Warmed IV fluids that areadministered at very low flow rates can cool as they flow down the IVtubing to the patient. Often this heat loss is ignored. One prior artapproach has been to simply heat the fluid to 41° C. to try to overcomethe loss for all flow rates.

In another aspect of fluid warming, most IV fluid warmers heat fluidthrough flexible plastic walls. Because these walls are inefficient intransferring heat, heaters are exposed to both sides of the disposableset. This requires inserting the disposable set into a slot or hingedclam shell configuration. Both of these designs do not allow the user tosee the fluid passing through the heat exchanger. As these plastic wallsare flexible, the pressure necessary for good heat transfer cannot beguaranteed as it is supplied only by the gravitational force of the IVfluid bag height. Cleaning of configurations with slots is difficult andtypically requires special tools or even disassembly, such as in thecase of blood spills.

One type of exemplary medical fluid warming system is described in US2005-0008354. In this device, fluid passes along a generally serpentinefluid flow path through a removable/disposable heat exchange body. Theheat exchange body is in thermal contact with a resistive film heatervia thermally conductive layers interposed between the heat exchangebody and the heater. Temperature sensors are provided that sense thetemperature of the heat exchange body and of the heater.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a fluid warming system includesa pair of sliding covers that maintains a removable heat exchange bodyin place in heat exchange communication with a heater assembly. The heatexchange body is pressed in place by the covers in the closed positionto guarantee good thermal conductivity. Fluid passing through the heatexchange body remains visible to the user. The heat exchange body iseasily inserted and removed by sliding the covers to a open position.The moving covers can double as user input mechanisms by turning thepower on and off and muting alarms. The covers do not cover anyindicators present on the heat exchange body. The covers can be readilyremoved for cleaning, and the covers can be used as a mountingmechanism.

In another aspect of the present invention, power to the heater can beincreased or decreased to adjust the fluid temperature to ensure thatthe fluid is at an appropriate temperature when it reaches the patient.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is an isometric view of a fluid warming device illustratingslidable covers in a closed position according to the present invention;

FIG. 2 is an isometric view of the fluid warming device of FIG. 1illustrating the slidable covers in a half closed position;

FIG. 3 is an isometric exploded view of the fluid warming device of FIG.1 with the slidable covers in an open position and a disposable setremoved;

FIG. 4 is an isometric view of a main body of the housing of the deviceof FIG. 1 with the slidable covers fully removed;

FIG. 5 is a plan view of the device of FIG. 2;

FIG. 6 is a cross sectional view taken along line A-A of FIG. 5;

FIG. 7 is a schematic view of a disposable set and heater assembly ofthe fluid warming device of FIG. 1;

FIG. 8 is a side view of a further embodiment of a fluid warming deviceillustrating gripping faces on the slidable covers;

FIG. 9 is a flow chart illustrating a system for adjusting power to aheater of a fluid warming device to accommodate heat loss in an IVtubing at low flow rates;

FIG. 10 is a graph illustrating heat loss across a disposable set heatexchange body and along a length of IV tubing and their combined heatlosses; and

FIG. 11 is a graph illustrating temperature loss for various lengths ofIV tubing and various flow rates.

DETAILED DESCRIPTION

A fluid warming device 10 according to the present invention isillustrated in FIGS. 1-7. The fluid warming device includes a housing 12having a main body 14 and two sliding covers 16. Within the housing,supported by the main body, are a removable heat exchange body 18 and aheater assembly 20. The sliding covers are independently slidable to aclosed position in which they retain the removable heat exchange body inplace, as described more fully below. The slidable covers are preferablyidentical.

The removable heat exchange body 18 and the heating assembly 20 areillustrated schematically in FIG. 7. The heat exchange body, also calleda disposable or removable set, includes an input port 22 connectable toan IV tubing line from a source of IV fluid, which may include aninfusion pump. The disposable set also includes an output port 24connectable to a further IV tubing line to deliver the IV fluid to thepatient. Within the disposable set, the IV fluid flows along a flow path(not shown) having a serpentine or other suitable configuration betweenthe input and output ports to optimize heat transfer to the fluid. See,for example, US Patent Pub. No. 2005-0008354, the disclosure of which isincorporated by reference herein. The disposable set is formed from anysuitable material, such as aluminum, to facilitate heat transfer to thefluid flowing therein. When inserted in the housing with the slidingcovers in a closed position, the disposable set is held in thermalcontact with the heater assembly, so that heat transfer from the heaterassembly to the disposable set causes heating of an IV fluid flowingtherethrough.

The heater assembly 20 is affixed within the main body 14 of the housing12. The heater assembly includes a heater 26 and one or more thermallyconductive layers 28, 30 interposed between the disposable set and theheater. Preferably, the heater is an electrically powered resistive thinfilm heater. A power line 32 to the heater from a suitable power sourceis provided. Alternatively, the device may include a battery compartmentor a connection to a battery pack, for example, for portable operation.Temperature sensors 34, 36 are provided that sense the temperature ofthe disposable set and of the heater. See, for example, US Patent Pub.No. 2005-0008354. The thermally conductive layers also electricallyinsulate the disposable set from the resistive heater. One thermallyconductive layer 28 may suitably comprise a phase transition material,and the other thermally conductive layer 30 may suitably comprise amaterial such as a graphite to optimize heat transfer between the heaterand the disposable set. See, for example, US Patent Pub. No.2005-0008354. It will be appreciated that other or further thermallyconductive layers may be provided. The main body 14 includes acompartment 38 on one side to receive the disposable set 18 in contactwith an exposed surface 40 of the uppermost thermally conductive layer30.

As noted above, the heat exchange body or disposable or disposable set18 is removable from the housing. The disposable set can be removed fromthe main body of the housing by sliding the two opposed sliding coversoutwardly in opposite directions. In this manner, the removable set canbe lifted out of the housing with the IV tubing still attached to theinput and output connectors, without breaking the fluid path. Fingercutouts 42 may be provided for ease of grasping the disposable set inthe main body.

Any suitable sliding mechanism to allow the covers to move axially intothe closed position can be provided. In the embodiment shown, the mainbody 14 of the housing includes protruding longitudinal tracks 46 alongtwo opposed longitudinal outer wall surfaces of the main body. See FIG.5. The sliding covers include complementary longitudinal recesses 48along inner wall surfaces that mate with the tracks and allow the coversto slide axially along the main body. When in the closed position, thesliding covers extend over the edges of the disposable set within therecess of the main body, thereby retaining the disposable set therein.See FIG. 1. The covers also compress the disposable set to the outermostthermally conducting surface 40 of the heater assembly. This compressionprovides the necessary pressure for proper heat transfer between theheater assembly and the disposable set. Preferably, the covers areretained in the closed position by frictional engagement with thedisposable set. Alternatively, any suitable latching or retainingmechanism may be provided.

Also, the covers 16 do not block the view of the bulk of the mid portionof the disposable set 18, allowing the operator to view the fluidpassing through the disposable set. The disposable set is also keyed tothe main body 14 in any suitable manner so that it fits within thecompartment in the correct orientation. For example, one end 47 of thedisposable set may be rounded to fit within an correspondingly roundedportion 49 of the compartment 38. The disposable set may include anarrow 50 thereon to provide an indication of the direction of flow, sothat the disposable set is inserted in the housing in the correctorientation. The covers do not block this arrow. Also, the main bodypreferably includes indicator lights, such as LEDs, thereon. Forexample, one LED 52 may provide an indication of temperature at theoutput port, and another LED 54 may provide an indication that theheater is connected to the power source. The covers do not block theseindicator lights either.

In one embodiment, the covers 16 can be maintained in two positions onthe main body or can be removed fully from the main body. While on themain body, the covers can be in a fully closed position or an openposition. The covers can include magnets or Hall Effect devices or otherproximity sensors that interface with a corresponding component withinthe main body to determine the positions of the covers and causeoperation of any appropriate switches. In a further embodiment, thecovers can be maintained in a third or intermediate, half closed,position on the main body, described further below.

More particularly, in the fully closed position, (see FIG. 1), thecovers 16 apply full pressure to the disposable set 18 to ensure goodthermal contact with the heater assembly. In this position, the slidingcovers can also be used to turn the power on to commence warming and/orto activate any audible or visible alarm(s). In the half closed position(see FIG. 2), the disposable set is still held in place by the covers,but warming is stopped, the audible alarm is silenced, and the visualindicators are turned off. The status LED could be flashed in batteryoperation to inform the user that the warmer is connected to the batteryand draining. When the covers are in the open position (see FIG. 3), thedisposable set 18 can be inserted and removed. No heating takes place,the audible alarm is silenced, and visual indicators are turned off. Thestatus LED could be flashed in battery operation to inform the user thatthe heater is connected to the battery and draining.

Any suitable latching or retaining mechanism can be provided to retainthe covers in the desired positions relative to the main body. Forexample, as shown in FIGS. 5 and 6, recessed surfaces 62 are provided onthe main body 14 that latch with corresponding tabs 64 on the covers inthe open position, preventing the covers from readily coming off themain body. Also, the tabs 64 abut surfaces 63 to hold the covers in theclosed position. Finger grips 68 are provided to aid in grasping thecovers to push or pull them to the desired position. The closed (andpower on) position can be indicated by arrows 70 and an adjacent “ON”marking on the covers. Similarly, the open (and power off) position canbe indicated by arrows 72 and an adjacent “OFF” marking on the covers.The covers can be fully removed from the main body in any suitablemanner, for example, by the insertion of a suitable tool, such as ascrew driver or dime, to lift the tab 64 over the surfaces 62.Alternatively, a latching or retaining mechanism can be configured torelease simply by the use of sufficient force. Removal of the coversallows the device to be readily cleaned. Alternatively, passageways inthe interior surfaces of the covers and a water tight main body housingallow cold sterilization by dipping in a sterilization fluid withoutcomplete removal of the covers.

Referring to FIG. 8, the sliding covers 14 may include opposed faces 74that include gripping teeth thereon to form gripping faces. The grippingfaces can be used to grip hospital clothing or bedding and hold thewarmer in place to reduce stress on the IV line when the covers arefully closed.

In another aspect of the present invention, power to the heater can beincreased or decreased to adjust the fluid temperature to ensure thatthe fluid is at an appropriate temperature when it reaches the patient.More particularly, some IV fluids that have been warmed are administeredat very low flow rates. These fluids cool as they travel down the IVtubing to the patient. The greater the difference between ambienttemperature and the fluid temperature, the greater the radiated heatlosses from the IV tubing.

A suitable controller is provided to perform the calculations andcommunicate with the heater to make the desired adjustments. Heaterpower is determined by the difference between a target temperature(typically in the range of 39 to 41° C.), and the actual fluidtemperature.

Referring to FIG. 9, the temperature drop across the heat exchanger iscalculated (step 102). This temperature drop is equal to the heaterpower divided by the thermal resistance of the heater assembly. Thethermal resistance can be readily determined by one of skill in the artfrom the thickness, thermal conductivity and area of the materialsbetween the heater and the fluid and stored as a constant.

Then, the controller calculates the temperature loss of the IV tubing tothe environment (step 104). First, the difference between the fluidtarget temperature and the ambient temperature is determined. Thetemperature loss is equal to this temperature difference multiplied bythe radiation loss and divided by the heater power. The ambienttemperature is measured by a suitable sensor located within the warmingdevice in close contact with the housing, which is very close to ambienttemperature. The radiation loss is a constant that is derived fromexperimentation with various lengths of the IV tubing and various flowrates. See FIG. 11.

Next, at step 106, the controller determines if the IV tubing loss isgreater than 1° C. Also at step 106, the controller also determines ifthe total drop along the IV tubing and across the heat exchanger isgreater than a drop limit. The drop limit is the maximum temperaturethat the fluid can be artificially raised so that the allowable surfacetemperature on the heat exchanger is not exceeded, for example, nogreater than 3° C. from the desired target temperature. If the answer atstep 106 is Yes, the actual fluid temperature is calculated at step 108as the measured fluid output temperature minus the drop limit. If theanswer at step 106 is No, the actual fluid temperature is calculated atstep 110 as the fluid output temperature in the IV tubing drop minus theIV tubing drop (from step 102) minus the heat exchanger drop (from step104). Using the calculated value of the actual temperature, heater poweris adjusted appropriately.

In this manner, heat loss along the IV tubing can be more efficientlycontrolled The system allows the fluid warming device to be located abit farther from the infusion site and still deliver normothermic fluid.

The invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims.

The invention claimed is:
 1. A fluid warming system to minimize heatloss through IV tubing to a patient, comprising: a) determining atemperature drop across a heat exchange body of an IV fluid warmingdevice; b) determining a temperature drop of the IV tubing to theenvironment, the IV tubing extending from the IV fluid warming device toa patient; c) determining if the temperature drop along the IV tubing isgreater than a determined temperature limit; d) determining if the totaltemperature drop along the IV tubing and across the heat exchange bodyis greater than a determined drop limit; e) if the determined valuesfrom step c) and step d) are greater than their respective limits,determining the actual fluid temperature as a fluid output temperatureminus the drop limit; f) if either of the determined values from step c)and step d) are not greater than their respective limits, determiningthe actual fluid temperature as a fluid output temperature minus theheat exchanger temperature drop; and g) adjusting power to the heatexchanger to achieve the desired output temperature.
 2. The fluidwarming system of claim 1, wherein the temperature drop across the heatexchanger in step a) is determined by dividing heater power of a heaterassembly that heats the heat exchange body by the thermal resistance ofthe heater assembly.
 3. The fluid warming system of claim 1, wherein thetemperature drop of the IV tubing in step b) is determined bydetermining the difference between a fluid target temperature andambient temperature, multiplying this difference by a radiation lossconstant to get a result, and dividing the result by heater power of aheater assembly that heats the heat exchange body.
 4. The fluid warmingsystem of claim 1, wherein the determined temperature limit of step c)is 1° C.
 5. The fluid warming system of claim 1, wherein the determineddrop limit of step d) is a maximum temperature to which the IV fluid canbe heated.
 6. The fluid warming system of claim 5, wherein the maximumtemperature is 3° C. above the desired temperature.
 7. The fluid warmingsystem of claim 1, wherein the fluid output temperature is measured by atemperature sensor on the IV fluid warming device.
 8. The fluid warmingsystem of claim 1, wherein the heat exchange body comprises a fluidinput port, a fluid output port, and a fluid flow path from the fluidinput port to the fluid output port, wherein the fluid input port isconnected to an IV fluid line to receive a fluid and the fluid outputport is connected to the IV tubing to deliver the fluid through the IVtubing to the patient.
 9. The fluid warming system of claim 8, whereinthe IV fluid warming device further comprises: a housing comprising amain body, a heat exchange body receiving compartment disposed in themain body, and a pair of covers axially slidable on the main body,wherein the heat exchange body is disposed in the heat exchange bodyreceiving compartment; and a heater assembly disposed within the mainbody, wherein the covers, when in a closed position, press the heatexchange body into heat exchange communication with the heater assembly.10. The fluid warming system of claim 9, wherein the covers are slidableto an open position from which the heat exchange body can be removedfrom or inserted into the heat exchange body receiving compartment, andwherein the heat exchange body can be removed from or inserted into theheat exchange body receiving compartment while attached to IV tubing.11. The fluid warming system of claim 9, wherein the covers include aswitch to activate power to the heater assembly when the covers are inthe closed position.
 12. The fluid warming system of claim 1, whereinfluid flowing through the heat exchange body is visible.
 13. A fluidwarming system to minimize heat loss through IV tubing to a patient,comprising: a) determining a temperature drop across a heat exchangebody of an IV fluid warming device, the heat exchange body comprising afluid input port, a fluid output port, and a fluid flow path from thefluid input port to the fluid output port, wherein the fluid input portis connected to an IV fluid line to receive a fluid and the fluid outputport is connected to the IV tubing to deliver the fluid through the IVtubing to a patient; b) determining a temperature drop of the IV tubingto the environment, the IV tubing extending from the IV fluid warmingdevice to the patient; c) determining if the temperature drop along theIV tubing is greater than a determined temperature limit; d) determiningif the total temperature drop along the IV tubing and across the heatexchange body is greater than a determined drop limit; e) if thedetermined values from step c) and step d) are greater than theirrespective limits, determining the actual fluid temperature as a fluidoutput temperature minus the drop limit; f) if either of the determinedvalues from step c) and step d) are not greater than their respectivelimits, determining the actual fluid temperature as a fluid outputtemperature minus the heat exchanger temperature drop; and g) adjustingpower provided to a heater assembly disposed in heat exchangecommunication with the heat exchange body to achieve the desired outputtemperature.
 14. The fluid warming system of claim 13, wherein the IVfluid warming device further comprises: a housing comprising a mainbody, a heat exchange body receiving compartment disposed in the mainbody, and a pair of covers axially slidable on the main body, whereinthe heat exchange body is disposed in the heat exchange body receivingcompartment, wherein the heater assembly is disposed within the mainbody, and wherein the covers, when in a closed position, press the heatexchange body into heat exchange communication with the heater assembly.